In a typical fueling transaction, a customer drives a vehicle up to a fuel dispenser in a fueling environment. The customer arranges for payment, either by paying at the pump, paying the cashier with cash, using a credit card or debit card, or some combination of these methods. The nozzle is inserted into the fill neck of the vehicle, and fuel is dispensed into the fuel tank of the vehicle. A display on the fuel dispenser indicates the amount of fuel that has been dispensed during the fueling transaction. The customer relies on the fuel dispenser to measure the amount of fuel dispensed accurately and charge the customer accordingly.
Operating internally within the fuel dispenser are valves that open and close the fuel flow path and a meter that measures the amount of fuel dispensed. The purpose of the meter is to accurately measure the amount of fuel delivered to the customer's vehicle so that the customer may be billed accordingly and fuel inventory updated. For pre-pay transactions, the fuel dispenser also relies on the meter to measure the fuel dispensed so as to control the termination of fuel dispensing.
Fuel dispenser meters may be positive displacement or inferential meters. Positive displacement meters measure the actual displacement of the fuel, while inferential meters determine fuel flow indirectly using a device responsive to fuel flow. In other words, inferential meters do not measure the actual volumetric displacement of the fuel. Inferential meters have some advantages over positive displacement meters. Chief among these advantages is that inferential meters may be provided in smaller packages than positive displacement meters. With either positive displacement or inferential meters, the meter generates a meter signal that is responsive to the amount of fuel flowing in the fuel flow path. The meter communicates the meter signal to a control system in the fuel dispenser.
One example of an inferential meter is described in U.S. Pat. No. 5,689,071, entitled “WIDE RANGE, HIGH ACCURACY FLOW METER.” The '071 patent describes a turbine flow meter that measures the flow rate of a fluid by analyzing rotations of turbine rotors located inside the fuel flow path of the meter. As fluid enters the inlet port of the turbine flow meter in the '071 patent, the fluid passes across two turbine rotors, which causes the turbine rotors to rotate. The rotational velocity of the turbine rotors is sensed by pick-off coils. The pick-off coils are excited by an alternating current signal that produces a magnetic field. As the turbine rotors rotate, the vanes on the turbine rotors pass through the magnetic field generated by the pick-off coils, thereby superimposing a pulse on the carrier waveform of the pick-off coils. The superimposed pulses occur at a frequency (pulses per second) proportional to the turbine rotors' rotational velocity and hence proportional to the measured rate of flow. The pulses are sent to a control system as meter signals in the form of pulser signals. The control system receives the meter signals from the meter and converts the meter signals into the fuel flow rate and the volume of fuel dispensed.
A problem may occur with accurately measuring fuel flow when a customer is fueling his or her automobile at a retail fuel dispenser. If a non-steady state condition occurs, for example, by the customer closing and opening the fuel nozzle in a rapid fashion, known as a “nozzle snap,” inaccuracy in fuel measured by the meter is introduced. The nozzle snap creates a pressure shock wave that causes a flow disturbance at the meter resulting in a false flow indication. If a flow switch is employed to detect when flow stops, the pressure shock wave causes the flow switch to bounce. The control system that receives the meter signals from the meter registers fuel flow without taking into account the flow disturbance. The cumulative effect of the nozzle snaps and the flow switch bouncing, if present, results in meter measurement inaccuracies. Meter measurement inaccuracies may cause the fuel dispenser displays to register false fuel flow rate and fuel volume dispensed, and may cause the accuracy to be outside of allowable limits.
Therefore, a need exists for a fuel dispenser to accurately measure fuel flow with a meter even during a nozzle snap or other non-steady state condition.